Functionalized 2,5‐Dipyridinylpyrroles by Electrochemical Reduction of 3,6‐Dipyridinylpyridazine Precursors

Abstract The ring contraction of pyridinylpyridazine derivatives into the corresponding pyrroles by electrochemical reduction was studied, and the influence of the substituents of the pyridazine precursors on the process is discussed. Cyclic voltammetry studies underlines the electron‐withdrawing or ‐donating effect of the substituent on the pyridazine ring, which determines the reaction pathway of their preparative electrolysis. The ring‐contraction process, with extrusion of nitrogen, proceeds by two subsequent two‐electron, two‐proton processes via a 1,2‐dihydropyridazine intermediate. The latter can either rearrange into an isolable 1,4‐dihydropyridazine or undergo formation of pyrroles by disproportionation or by a second electrochemical reduction involving two‐electrons and two protons. X‐ray structure, fluorescence spectra, and conformational analysis of pyridinylpyrrole sequences supported this study. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)